1. Field of the Invention
The invention relates to a method of positioning a combined magnetic read and write head in a magnetic coding storage device. The invention also relates to a system for performing this method.
2. Description of the Prior Art
Methods of positioning magnetic head in magnetic coding storage devices, more particularly magnetic tape devices, are known in the art.
A method of this kind for the accurate positioning of a magnetic head of a magnetic coding storage device is described in EP-A1-0 276 451. In this known method, a stepping motor is used to move the magnetic head at right angles to the direction of movement of the recording medium, which is a magnetic tape. To control the stepping motor a correction value which corresponds to the offset between a write head and a read head of the magnetic tape along one track of the recording medium is added to a position value associated with each track of the recording support. Using this known method it is possible to achieve an accurate positioning particularly if the write and read head or in the magnetic head are not accurately aligned along one track, or if the magnetic head itself has not been accurately adjusted.
In the recording of data signals on a recording medium, the number of tracks is very important to the storage density. It is therefore very important to accommodate the largest number of tracks on a medium, e.g. a magnetic tape. A high track density, however, can be achieved only if there is high accuracy of positioning.
For this purpose, it is known from JP-A-59 185 020 to adjust the position of a multi-channel magnetic head with respect to a magnetic tape by providing, in addition of the write/read heads for the individual recording tracks, other write/read heads for recording or reading reference tracks. Pairs of write and read heads for recording and reading edge tracks are provided in each case opposite the two tape edges. A write/read head is disposed in the center of the multi-channel magnetic head for recording a center reference track.
For recording, the multi-channel magnetic head is first roughly positioned. The edge heads are used in each case to record and then read an auxiliary track at the edges of a magnetic tape. The off-position of the magnetic head can be corrected by shifting the magnetic head transversely to the direction of the track until the read signals of both edge tracks have the same amplitude. The multi-channel magnetic head is then accurately positioned with respect to the magnetic tape. During the information recording, the central reference track is recorded at the same time. On subsequent reading of the magnetic tape the magnetic head is aligned and kept aligned exactly on the center of the central reference track and hence to the tape center by means of the centrally disposed auxiliary read head, which is divided in half. This known system is relatively complex and can also be used rationally only in multi-track heads. It also has the disadvantage that tolerance build-ups which occur precisely with multi-track heads are harmless only if recorded magnetic tapes are again read with the same magnetic head which was used to record the tape.
A similar solution is also disclosed in JP-A-59 231 731 in which an auxiliary write head is used to record a reference track near the tape edge. The reference track being evaluated by a two-part auxiliary read head in order to position a read head system.
During normal tape transport, of course, additional tape movements occur vertically to the direction of tape transport. This is known as tape wander. A positioning device for a multi-track head is disclosed in JP-A-58 189 877 for countering this disturbing effect. In this magnetic head, a centrally disposed auxiliary write head is again used to record a reference track, which is read during reading by means of a two-part read head. The read signals are evaluated to derive a positioning signal with which the multi-track head is continuously kept at the center of the reference track despite the tape wander.
Finally, EP-A 1-32 660 discloses a track following control for a multi-track head, two auxiliary read heads being provided. These two heads are disposed laterally on the multi-track head at an angle to the direction of tape transport at opposite edges of the tape. These auxiliary read heads detect edge tracks recorded with the multi-track head. The flux changes recorded in the information tracks by the multi-track head are aligned vertically to the longitudinal direction of the magnetic tape. If the multi-track head is not aligned exactly to the center of the magnetic tape during reading, the auxiliary read head gaps pass over these vertical flux changes in the edge tracks at somewhat different times. Thus the phase difference in the read signals of the auxiliary read heads is an index of the offset of the multi-track head from the tape center. By means of feedback control for the vertical position of the multi-track head, the attempt is made to cancel out this phase difference. The disadvantage of this known solution is that the angles of incidence of the auxiliary read heads must be made large enough to allow good resolution for positioning control. This is usually not possible, however, because as the angles of incidence of the auxiliary read heads increase so their read signal amplitudes continue to decrease.